With the increased focus on vehicle emissions, exhaust gas recirculation (“EGR”) is utilized in many conventional internal combustion engines to assist in the reduction of throttling losses at low loads, to improve knock tolerance, and to reduce the level of oxides of nitrogen (“NOx”) in the exhaust gas at high engine loads. EGR is especially important as an emissions reducer in internal combustion engines that run lean of stoichiometry and thereby are prone to emitting higher levels of NOx emissions.
One proposition that has been considered in the construction of internal combustion engine systems is to utilize one or more of a plurality of cylinders as a dedicated source of EGR. For example, in an engine having two or more cylinders, the entire supply of exhaust gas produced in one of the cylinders is transferred to the intake ports of the other cylinders as EGR. In engines having greater numbers of cylinders (e.g., 4, 6, or 8 cylinders), timing considerations may cause it to be advantageous to dedicate up to half of the cylinders (i.e., 2, 3, or 4 cylinders) to the production of EGR.
A disadvantage to this type of internal combustion engine system is that an internal combustion engine that dedicates the use of one or more cylinders to production of EGR may not deliver EGR uniformly to the remaining cylinders. For example, the cylinder event following the dedicated EGR cylinder event may be prone to receive more EGR diluent than the subsequently firing cylinders. These variations in cylinder makeup (i.e. combustion air, fuel and EGR diluent) can result in uneven combustion performance that is difficult to control over a broad range of operating conditions. In addition, engines having displacements that are uniform among the cylinders, may be incapable of precisely delivering desired quantities of EGR.
To at least partially address these disadvantages, a number of configurations are being studied, including configurations wherein more than one in four cylinders operates as a dedicated EGR cylinder or where a dedicated EGR cylinder produces more than a single volume of exhaust gas for every four volumes of exhaust gas produced by other cylinders or where displacements of dedicated EGR cylinders differ from displacements of remaining cylinders. To enable such configurations, it would be advantageous to have a crankshaft that can facilitate improved distribution of EGR among non-EGR cylinders. It would also be advantageous to have a crankshaft that can enable cylinders displacements that differ between the EGR and non-EGR cylinders.